The ornamental design for a fin for a watersports board, as shown and described.
FIG. 1 is a perspective view of a fin for a water sports board showing my new design;
FIG. 2 is a right elevational view thereof;
FIG. 3 is a left elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is bottom plan view thereof.
The broken line showing of the surfboard and the second fin in FIG. 1 and the upper portion of the fin in FIGS. 1–5 is for illustrative purposes only and forms no part of the claimed design.